In the electronics industry screen printing is used to make various components. For example, microelectronic packages called hybrids, or multi-chip modules, utilize circuits and electrical devices made by screen printing. Conductive, resistive and dielectric patterns of a circuit can be formed by screen printing onto a rigid substrate such as a circuit board. Screen printing is also used in the fabrication of field emission displays (FEDs) for flat panel displays. With a FED, the normal topography used to make a hybrid can be multi level due to the gap which is required between the anode and cathode components of the display. This places some additional demands on the screen printing process.
Screen printing for microelectronics is similar to the method used to make t-shirts and printed panels for industrial equipment but at the high end of the technology. A typical screen printing process for a multi level hybrid would be to print a conductive layer, dry the layer and fire. The substrate would then be processed with the next layer, usually a dielectric composition. After dry and fire another layer of conductor would be fired.
With screen printing, a screen is used to deposit a thick-film paste, or other printing material, onto a substrate (e.g., polyimide circuit board, silicon baseplate). Different techniques are used to transfer the desired pattern from a mask containing artwork to the screen.
To produce a screen, a stainless steel or monofilament polyester screen mesh is stretched and attached to a metal frame. A negative pattern must then be generated on the mesh so that the printing material can be forced through the screen to produce a positive pattern for the substrate. A photosensitive emulsion can be used to make the negative pattern on the screen. There are three methods for the application of the emulsion to the mesh: direct, indirect and indirect-direct.
In the direct emulsion method, the emulsion is initially applied to the mesh in a viscous state and then dried. After drying, the emulsion is exposed through a mask using UV light, and then developed under a water jet to form a patterning layer on the mesh. For a negative acting photosensitive emulsion, the exposed regions of the emulsion are polymerized and the mesh is sealed in these regions. Conversely, the unexposed regions of the emulsion are washed away and form open areas on the mesh. For a positive acting photosensitive emulsion, the exposed portions are removed and the unexposed portions of the emulsion are left.
For screen printing the pattern defined by the patterning layer onto a substrate, the substrate is secured to a support platform within a screen printer. The screen is mounted within the screen printer, parallel to the substrate but spaced apart from the substrate with a slight gap. The printing material is then applied to the screen and a squeegee (e.g., rubber blade) is moved across the screen at a constant rate. The squeegee forces the printing material through the open areas of the screen and prints the pattern defined by the patterning layer onto the substrate.
For printing small closely spaced features, fine mesh screens are preferred. The screen mesh count refers to the number of screen openings per linear inch. The width of a screen opening is related to the mesh count and to the diameter of the screen wire by the formula EQU W.sub.o= (1-DM)/M
where
W.sub.o is the width of the screen opening in inches PA1 D is the diameter of the screen wire in inches PA1 M is the mesh count
By way of example a commercially available 400 mesh screen has a wire diameter of about 0.75 mil (19.05 .mu.m). A 400 mesh screen is formed with square openings that have a width of about 1.75 mil (44.45 .mu.m).
One problem with the screen printing of patterns having small closely spaced features, is that the resolution and spacing of the features of the pattern can be adversely affected by the screen wires. In particular, with a pattern having feature sizes approximately equal to the size of the openings in the mesh, the resolution and spacing of the features can be distorted by the screen wires.
For example, features that are about the size of the screen openings (e.g., 1.75 mils in diameter for a 400 mesh screen) require open areas in the patterning layer that are approximately the same size as the screen openings. If a 0.75 mil (19.05 .mu.m) screen wire intersects an open area of the patterning layer, then the open area is either partially blocked or split into two smaller openings by the screen wire. For a complex pattern with many features these intersections can occur at many places. In general, the interference of the screen wires with the open areas of the patterning layer, distorts some of the features that are transferred onto the substrate. Because of this distortion, conventional screen printing processes cannot be used to successfully print feature sizes that are less than about 4 mils (101.6 .mu.m) in size and spacing.
Another limitation of screens formed for screen printing occurs during the development of the photosensitive emulsion which forms the patterning layer. During a development step, the unexposed material for a negative emulsion, or the exposed material for a positive emulsion, must be cleared from the screen. Clearing out this material is complicated by the presence of the screen wires. Consequently, if the material is not completely cleared, pattern defects can occur.